[s-cars] Waste gate creep

[Robert Pastore]

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Hap:

A few questions, comments, & probably useless opinions, FWIW:
<snip>
"Another related issue for my car has been WG float. This particular
problem
comes from using a significantly larger(Happersize tm.) turbo and still
using
the RS2 EM. The pre-turbine pressure from using this larger turbo can
build
to a very high figure. The float manifests itself as a 2-4 psi boost
pressure
pulse that can be eliminated with a higher WG spring preload."
<snip>

I completely agree that the rs2 EM is too small for the CFM you are
pushing.   I do not know what your hot side trim is, but it sounds to me
like the hot side of your turbo is too small also.    If you have a TEC
turbo, I recall they are partial to a t3 stage III, which has an exducer
bore only marginally larger than a rs2.  Innovative, on the other hand,
typically specifies a t3 stage V for most high HP I-5 Audis.   The
symptoms of an undersized turbine section seem to match perfectly with
your set of problems:

I've forgotten most of what I learned in Physics class, but I think it
was Boyle's law that said "P1*V1=P2*V2".  Basically, as you force a
certain mass of air into a given volume, the pressure will increase as
volume decreases.   Pretty simple stuff.   So if you have already
measured pre-turbo pressures significantly higher than the stock
pressure ( please share the data, as I have never measured either and
would love to get an idea of what a factory baseline is), I think the
data is telling you that the exducer bore is too small, with the result
will be spooling up the turbo very early, and then over spinning it at
higher engine rpms.    The ramifications on the cold side will be
surging at low rpms, and poor efficiency range at higher rpms as the
compressor is spinning too fast.

Since the s4/s6 uses boost pressure in the lower chamber acting against
spring pressure in the upper chamber, this early, strong, boost "attack"
could easily cause WG creep as the stock WGFV is probably not up to the
task of bleeding off boost from the lower chamber fast enough. A top
chamber boost controller doesn't solve the root problem, but should help
mask the symptoms.  But I am not sure I agree with or maybe I don't
understand your statement about "limiting versus controlling boost" with
the AVC-R.   The top chamber can only help keep the WG shut, but can't
do a thing about opening it.  It is the pressure differential across the
WG diaphragm that determines whether the WG is open or closed.   On the
bottom side, you have EM pressure pushing against the face of the valve
(probably a small factor since the areas is not that large) and boost
pressure in the lower chamber.  The stock system uses a WGFV to regulate
the amount of boost seen in the bottom chamber. If you are using a top
chamber boost controller, you should ditch the WGFV and let the bottom
chamber see full boost.   The bottom chamber pressure acts against the
WG spring pressure + atm. pressure only (in the stock system), and WG
spring plus regulated boost with an aftermarket boost controller. I
don't think you can set up an aftermarket boost controller and have the
benefit of the higher pressure differential across the WG diaphragm,
while retaining the WGFV, which is the only tool the ECU has to provide
the "override protections".


I also recall you've invested in/investigated several solutions to cure
what you thought was a compressor surge problem --i.e. a big bypass
valve, dual bypass valves, and streamlining the intake tract before the
turbo (as per Julian Edgar).    My guess is that you properly identified
compressor surge, but maybe didn't couple the surge with the high EM
pressure to conclude that the turbine was over spinning because the hot
side is too small.   With a properly sized hot-side, the EM pressure
will be lower (and so will temps), the cylinders will breath better, and
the WG will be used much less -- i.e. more energy can go into spooling
and driving the turbine, and less "wasted" through the WG and out the
tailpipe.


Bob